The present study was aimed at evaluating early changes in glycolysis, the redox state of free cytosolic NAD(P)-couples, and the adenine nucleotide system in lens in both control and 50% galactose-fed rats, with the possibility of preventing these with an aldose reductase inhibitor (ARI). Experiments were performed on male Sprague-Dawley rats fed the galactose diet for 2-14 days. The levels of glucose, galactose, glycolytic intermediates, (-glycerophosphate, malate, NAD, ATP, ADP, AMP were assayed spectrofluorometrically in individual lenses by enzymatic procedures, while galactitol and myo-inositol were quantified by GC-MS. Depressed glycolysis in lens in galactose-fed rats is consistent with decreased NAD+/NADH and adenine nucleotide phosphorylation. Early changes in lens glucose utilization, redox state of NAD-couples, and energy metabolism in this model of galactosemia are similar to those in diabetes, are at least in part mediated by aldose reductase involved mechani sms, and can be partially prevented by an aldose reductase inhibitor.